The present invention relates to an image forming method, an image forming apparatus and a recording medium for use in the method and the apparatus, which enable a printer, a copying machine or the like employing a simultaneous transferring and fixing method, with which a toner image formed by an indirect electrophotographic method is transferred to the surface of a recording medium and the toner image is fixed to the recording medium when the toner image is transferred, to have an excellent glossiness and image quality.
In recent years, a digital indirect electrophotographic method of forming a color image has been widely used. In general, the digital indirect and dry electrophotographic method is structured in such a manner that yellow, magenta, cyan and black toners each of which has been prepared by mixing a coloring matter, such as pigment and/or dye, with thermoplastic resin is electrostatically held on the surface of a photosensitive member, which is an image carrier which has been digitally addressed with exposing light formed by converting image information into light information so that a toner image is formed; the toner image is electrostatically transferred to a recording medium; and the toner image is melted and fixed with heat and pressure so that an image is formed.
In general, the toner for use in the indirect and dry electrophotographic method has particle sizes of 5 .mu.m to 12 .mu.m, and each color toner is transferred to the recording medium by a weight of 0.3 mg/cm.sup.2 to 1.2 mg/cm.sup.2. The thermoplastic toner is transferred to one- to four-layered recording medium. When the toner is melted with heat, it is softened and brought into a molten state. However, the overall body of the toner does not fully penetrate into the paper, which is a usual recording medium even after the toner has been heated and pressed. The toner swells by about 5 .mu.m to about 20 .mu.m over the recording medium.
The relationship between the area ratio of the thus-formed image, that is, input to the recording medium and gloss of the image is shown in FIG. 1. FIG. 1 shows results of measurement of 75.degree.-specular gloss of images formed by a method conforming to JIS P8142 in such a manner that magenta images are formed on cast coat paper (enamel coat/Yonago Kako Seishi Kabushiki Kaisha) which is high glossy paper, J-coat paper (Fuji Xerox) which is medium glossy paper and J-paper (Fuji Xerox) which is low glossy and non-coated plain paper are used as the recording mediums by using a multi-line screen such that the input image area ratio is varied. FIG. 2 shows an image profile of a portion of an image which has been formed on the cast coat paper as described above, which is obtained by a 3D surface roughness meter and in which the image area ratio is 40%. As can be understood from FIGS. 1 and 2, solid image portions have a relatively high glossiness. However, incident light is scattered considerably in halftone regions and highlighted regions because the multilines and dots are in the form of convex over the recording medium. Therefore, if an image, such as a image of a human being or the like, having relatively steep gradient is formed, an image having high gloss regions and low gloss regions mixed with one another is formed, thus resulting in a sense of incompatibility being allowed to arise. A fact is known that a rough surface image of the foregoing type suffers from unsatisfactory color reproducibility owning to an influence of irregular reflection on the surface of the image and thus an image having insufficient visibility is formed. Another fact is known that an image having a rough surface and formed on a transparent recording medium is, in a case of an projection operation using an OHP, encounters deterioration in the color developing characteristic.
To improve the quality of a color image of the foregoing type, a method has been suggested in the Unexamined Japanese Patent Application Publication No. Sho 63-92965, in which a transparent resin layer is formed on a recording medium; toner is transferred to the recording medium; and then a roll type heat fixing unit is operated so that the toner is embedded in the transparent resin layer. According to this method, the roughness of the image can somewhat be prevented. An influence of application of silicon oil as a releasing agent to the fixing heat roll as has been performed with the conventional method results in formation of a silicon oil film having a low surface tension between the toner and the transparent resin. As a result, the toner cannot sufficiently be embedded in the transparent resin layer and the toner image portion projects upwards over the surface of the recording medium.
In, for example, the Unexamined Japanese Patent Application Publication No. Hei 5-216322, a method having a similar object has been suggested in which toner is electrostatically transferred to a recording medium having, on the surface thereof, a transparent resin layer composed of thermoplastic resin having a thickness of 20 .mu.m to 200 .mu.m; and then a belt-type fixing unit is operated so that the toner is embedded in the transparent resin layer. Since the belt-type fixing unit is able to separate toner from the belt after the toner has been fixed to the recording medium, the autoagglutination force of the toner can be used as the force for preventing adhesion to the belt. As a result, any releasing agent is not required and sufficiently long heating time can be permitted. Therefore, surface roughness can be moderated. However, the disclosed types of the toner resin and the thermoplastic resin in the surface layer cannot attain satisfactory compatibility between the molten toner resin and the thermoplastic resin in the surface layer of the recording medium. As a result, a fact was found that difference in the refractivity arose at the interface in the coated surface layer, thus causing the color reproducibility to deteriorate and somewhat irregular surface to remain.
Each of the disclosed image forming methods, having the step of electrostatically transferring a toner image to a recording medium and using the thermoplastic resin formed on the surface of the recording medium and having a low dielectric constant, suffers from problems of color irregularity and narrowing of the color producible region because the transfer rate of a color, which is finally transferred, is reduced when, for example, color toners are transferred in a multiple manner.
To solve the problem which arises when an image is transferred because of deterioration in the dielectric constant, in the Unexamined Japanese Patent Application Publication No. Hei. 5-273781, a recording method has been suggested which has a structure that inorganic oxide particles are dispersed and contained in thermoplastic transparent resin so that the dielectric constant is raised. However, paper employed as the base of a recording medium has a non-uniform structure in general from a microscopic viewpoint. Therefore, disorder of the electric field occurring attributable to electrical irregularity of the recording medium when the transference is performed cannot easily be prevented. Thus, there arises a problem in that color irregularity is generated and the graininess deteriorates.